Multilayered ceramic structures of this type contemplated by the present invention are generally produced by placing in superposition at least two molded sheets containing a ceramic powder and a binder (these sheets are usually referred to as "ceramic green sheets") to prepare a laminate and burning the laminate. The interface between the molded sheets is subjected to a suitable treatment, such as screen printing of an electroconductive powder paste, in accordance with the intended use of the multilayered ceramic structure to be finally obtained.
In this production process, in preparing a laminate by superposing the molded sheets, the molded sheets superposed on each other to form a unitary laminate have to be bonded together by appropriate means so as to prevent a positional deviation between the sheets during handling prior to the burning step (e.g., punching the laminate into a predetermined shape) or during the burning step. To attain this purpose, one of the following bonding methods has been commonly employed: An adhesive polymer is dissolved in an organic solvent and the so prepared adhesive solution is applied to the opposing surfaces of molded sheets by a suitable coating means such as brushing or rollers; or the molded sheets are bonded into a unitary laminate by applying a suitable degree of pressure at temperatures not lower than the softening point of a binder in the sheets.
However, the first method involves difficulty in forming a thin uniform coating and partial delamination or positional deviation is likely to occur between sheets when the laminate is punched into a certain shape. Unevenness in the thickness of coating causes an additional disadvantage in that the thicker portion of the adhesive coating will lead to the formation of voids or blisters between the sheets at burning and the interlaminar adhesiveness of the sheets will be greatly impaired to eventually cause significant deterioration in the quality and yield of the finally produced multilayered ceramic structure. As a further problem, the adhesive solution has to be applied manually and great care must be exercised during coating operations in order to minimize the occurrence of the troubles described above. As a result, it has been difficult to fully automate the manufacturing process including lamination, punching and burning if the individual sheets are bonded by application of an adhesive solution.
The second method which employs hot pressing requires very fine control of temperature and pressure; if either temperature or pressure is insufficient, poor adhesion will result, and if they are excessive, troubles will occur such as the deformation of molded sheets and discontinuity of the electrical path formed of a conductive powder paste.